In the fields of displays for use in television receivers and information terminals, studies have been made for replacing conventionally mainstream cathode ray tubes (CRT) with flat-panel displays that are to comply with demands for a decrease in thickness, a decrease in weight, a larger screen and a high fineness. Such flat panel displays include a liquid crystal display (LCD), an electroluminescence display (ELD), a plasma display panel (PDP) and a cold cathode field emission display (FED). Of these, a liquid crystal display is widely used as a display for an information terminal. For applying the liquid crystal display to a floor-type television receiver, however, it still has problems to be solved concerning a higher brightness and an increase in size. In contrast, a cold cathode field emission display uses cold cathode field emission devices (to be sometimes referred to as a “field emission device” hereinafter) capable of emitting electrons from a solid into a vacuum on the basis of a quantum tunnel effect without relying on thermal excitation, and it is of great interest from the viewpoints of a high brightness and a low power consumption.
FIGS. 28 and 29 shows a cold cathode field emission display to which the field emission devices are applied (to be sometimes referred to as a “display” hereinafter). FIG. 28 is a schematic partial end view of the display, and FIG. 29 is a schematic partial perspective view of a cathode panel CP when the cathode panel CP and an anode panel AP are disassembled.
The field emission device shown in FIG. 28 is a so-called Spindt-type field emission device having a conical electron-emitting portion. Such a field emission device comprises a cathode electrode 11 formed on a supporting member 10, an insulating layer 112 formed on the supporting member 10 and the cathode electrode 11, a gate electrode 13 formed on the insulating layer 112, an opening portion 117 formed through the gate electrode 13, an opening portion 118 formed through the insulating layer 112, and a conical electron-emitting portion 19 formed on the cathode electrode 11 positioned in the bottom portion of the opening portion 118. Generally, the cathode electrode 11 and the gate electrode 13 are each formed in the form of a stripe in directions in which the projection images of these two electrodes cross each other at right angles. Generally, a plurality of field emission devices are arranged in a region (corresponding to one pixel, where the region will be called an “overlap region” or an “electron-emitting region EA” hereinafter) where the projection images of the above two electrodes overlap. Further, generally, such electron-emitting regions EA are arranged in the form of a two-dimensional matrix within an effective field (which works as an actual display portion) of the cathode panel CP.
The anode panel AP comprises a substrate 30, a phosphor layer 31 (31R, 31B and 31G) which is formed on the substrate 30 and has a predetermined pattern, and an anode electrode 34 formed thereon. A black matrix 32 is formed on the substrate 30 between one phosphor layer 31 and another phosphor layer 31, and a separation wall 33 is formed on the black matrix 32.
Each pixel is constituted of a group of the field emission devices formed on the electron-emitting region EA which is an overlap region of the cathode electrode 11 and the gate electrode 13 of the cathode panel side and the phosphor layer 31 of the anode panel side arranged so as to face the electron-emitting region EA. In the effective field, such pixels are arranged on the order, for example, of hundreds of thousands to several millions.
The anode panel AP and the cathode panel CP are arranged such that the electron-emitting regions EA and the phosphor layers 31 are opposed to each other, and the anode panel AP and the cathode panel CP are bonded to each other in their circumferential portions through a frame 35, whereby the display is produced. In an ineffective field which surrounds the effective field and where a peripheral circuit for selecting pixels is formed, a through-hole (not shown) for vacuuming is provided, and a tip tube (not shown) is connected to the through-hole and sealed after vacuuming. That is, a space surrounded by the anode panel AP, the cathode panel CP and the frame 35 is in a vacuum state.
A relatively negative voltage is applied to the cathode electrode 11 from a cathode-electrode control circuit 40, a relatively positive voltage is applied to the gate electrode 13 from a gate-electrode control circuit 42, and a positive voltage having a higher level than the voltage applied to the gate electrode 13 is applied to the anode electrode 34 from an anode-electrode control circuit 43. Between the anode electrode control circuit 43 and the anode electrode 34 is generally provided a resistance member R0 (having a resistance value of 1 MΩ in a shown example) for preventing an over-current and discharging.
When such a display is used for displaying on its screen, a scanning signal is inputted to the cathode electrode 11 from the cathode-electrode control circuit 40, and a video signal is inputted to the gate electrode 13 from the gate-electrode control circuit 42. Due to an electric field generated when a voltage is applied between the cathode electrode 11 and the gate electrode 13, electrons are emitted from the electron-emitting portion 19 on the basis of a quantum tunnel effect, and the electrons are attracted toward the anode electrode 34 and collide with the phosphor layer 31. As a result, the phosphor layer 31 is excited to emit light, and a desired image can be obtained. That is, the working of the display is controlled, in principle, by a voltage applied to the gate electrode 13 and a voltage applied to the electron-emitting portion 19 through the cathode electrode 11.
In the field emission device having the above structure, electrons are emitted from the electron-emitting portion 19 with certain angles from the normal of the electron-emitting portion 19. As a result, electrons emitted from the electron-emitting portion 19 may not collide with the opposed phosphor layer 31 but collide with a phosphor layer 31 adjacent to the opposed phosphor layer 31. When such a phenomenon takes place, the brightness is decreased and an optical crosstalk occurs between contiguous pixels.
For preventing the occurrence of the above phenomenon, there has been proposed a field emission device provided with a focus electrode 215 as shown in the schematic partial end view of FIG. 30. In this field emission device, a second insulating layer 214 is further provided on a gate electrode 13 and a first insulating layer 212, and the focus electrode 215 is formed on the second insulating layer 214. The focus electrode 215 has the form of one sheet covering the effective field. Reference numeral 216 shows a first opening portion formed through the focus electrode 215 and the second insulating layer 214, reference numeral 217 shows a second opening portion formed through the gate electrode 13, and reference numeral 218 shows a third opening portion formed through the first insulating layer 212. A relatively negative voltage (for example, 0 volt) is applied to the focus electrode 215 from a focus-electrode control circuit 41. When provided, the focus electrode 215 can converge the path of electrons that are emitted from the first opening portion 216 toward the anode electrode 34. Between the focus electrode 215 and the focus-electrode control circuit 41 is provided a resistance element R.
Meanwhile, in the above display, the distance between the anode panel AP and the cathode panel CP is approximately 1 mm at the largest, so that an abnormal discharge (spark discharge) is liable to take place between the field emission device of the cathode panel (more specifically, the focus electrode 215) and the anode electrode 34 of the anode panel AP.
In a mechanism in which a discharge takes place in a vacuum space, first, electrons and ions that are emitted from the field emission device under a strong electric field work as a trigger to cause a small-scaled discharge. And, energy is supplied to the anode electrode 34 from the anode-electrode control circuit 43, the anode electrode 34 is locally temperature-increased, and an occluded gas inside the anode electrode 34 is released, or a material constituting the anode electrode 34 is caused to vaporize, so that the small-scaled discharge presumably grows to be an abnormal discharge. Besides the anode-electrode control circuit 43, energy accumulated in an electrostatic capacity formed between the anode electrode 34 and the field emission device may possibly work as a source for supplying energy that promotes the growth to the abnormal discharge.
When the above abnormal discharge takes place, not only the display quality is extremely impaired, but also the anode electrode 34 and the field emission device are damaged. That is, when the above abnormal discharge takes place, the potential in the focus electrode 215 comes close to the potential in the anode electrode 34, and the potential in the focus-electrode control circuit 41 connected to the focus electrode 215 is also increased, so that the focus-electrode control circuit 41 may be damaged. Further, as a result of the fact that the potential in the focus electrode 215 comes close to the potential in the anode electrode 34, the potential in the gate electrode 13 is also increased. As a result, the potential difference between the gate electrode 13 and the electron-emitting portion 19 is increased. Therefore, excess electrons may be emitted from the electron-emitting portion 19, the electron-emitting portion 19 may be damaged, or the gate-electrode control circuit 42 connected to the gate electrode 13 may be damaged. Further, the potential in the cathode electrode 11 is increased, and as a result, the cathode-electrode control circuit 40 connected to the cathode electrode 11 may be damaged.
FIG. 31 shows an equivalent circuit when an abnormal discharge takes place between the focus electrode 215 and the anode electrode 34. In this case, the voltage (VA) applied to the anode electrode 34 is 5 kV, and the voltage applied to the focus electrode 215 is 0 volt. Due to the abnormal discharge between the anode electrode 34 and the focus electrode 215, a discharge current i flows. An imaginary resistance value (r) between the anode electrode 34 and the focus electrode 215 in this case is assumed to be 10 Ω. A resistance element R provided between the focus electrode 215 and the focus-electrode control circuit 41 is assumed to have a resistance value of 1 kΩ. Further, an electrostatic capacity CAF based on the anode electrode 34 and the focus electrode 215 is assumed to be 60 pF. FIG. 32 shows the result of simulation of a change in potential at a point “A” in FIG. 31. As is clearly shown in FIG. 32, the potential at the point “A” (i.e., the potential in the focus electrode 215) comes to be 2.5 kV at its maximum.
For inhibiting the abnormal discharge (spark discharge), it is effective to control the emission of electrons and ions which trigger the discharge, while it is required to control the particles extremely strictly therefor. In a general production process of the cathode panels CP or the display panels using the cathode panels CP, practicing the above control involves great technical difficulties.
Therefore, it is an object of the present invention to provide a cold cathode field emission display in which an abnormal increase in potential in the focus electrode can be suppressed even when an abnormal discharge takes place.